Bimodal HDPE comprises a low molecular weight (LMW) fraction and a high molecular weight fraction (HMW), typically referred to as a bimodal molecular weight distribution (MWD). Bimodal HDPE combines the advantages of low molecular weight polyethylene such as ease of processing and high melt flow index with the physical property advantages of high molecular weight polyethylene such as good impact resistance and good slow crack growth resistance. In making bimodal HDPE, the relative proportion of the low and high molecular weight fractions may be adjusted (as measured by the MWD) to provide HDPE having desired physical properties. For example, broadening the MWD of an HDPE polymer typically tends to improve the shear response of the polymer, thereby improving processing behavior in extrusion processes (such as in blown film, sheet, pipe and blow molding equipment).
The MWD may be determined by means of a curve obtained by gel permeation chromatography (GPC). For a polymer having a bimodal MWD, the GPC curve may resemble a peaked bell curve having a “shoulder” on the high molecular weight side of the peak or by two distinct peaks. Generally, the MWD is defined by a parameter known as the polydispersity index (D), which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn), i.e., D=Mw/Mn. The polydispersity index (D) provides a measure of the width of the molecular weight distribution for a polymer composition.
Bimodal HDPE is typically produced in a multi-stage polymerization process, for example polymerization of the low molecular weight fraction in a first stage and polymerization of the high molecular weight fraction in a second stage (or it can be the reverse). The multi-stage polymerization may be carried out in a single reactor or in two or more reactors in series, and suitable reactor types include stirred tanks, loop reactors, gas phase reactors, tubular reactors, autoclaves, and combinations thereof. Differing polymerization conditions may be achieved in the stages by varying parameters such as the reaction conditions (e.g., time, temperature, pressure, etc.) and the type and amount of reactants (e.g., monomer, co-monomers), catalysts, cocatalysts, chain transfer/termination agents (e.g., hydrogen), and the like.
Conventional, bimodal HDPE polymerization processes typically use one or more Zeigler-Natta catalysts wherein the production of low molecular weight and high molecular weight fractions is achieved by adjusting the hydrogen response of the Zeigler-Natta catalyst—an increase in hydrogen response producing a lower molecular weight polymer and conversely a decrease in hydrogen response producing a higher molecular weight polymer. More specifically, hydrogen serves as a chain termination agent for the Zeigler-Natta catalysts. Increasing the concentration of hydrogen in the polymerization reaction leads to increased termination of the polymer chains (i.e., shorter chain lengths), which produces a lower molecular weight polymer. Conversely, decreasing the concentration of hydrogen in a polymerization reaction leads to decreased termination of the polymer chains (i.e., longer chain lengths), which produces a higher molecular weight polymer. A common way of adjusting the hydrogen concentration (and thus the hydrogen response) in a conventional bimodal polymerization process is to vent hydrogen from a reactor, which wastes valuable reactants such as monomers and results in increased operating costs. The present invention provides an improved process for production of bimodal HDPE wherein the wasteful venting of hydrogen is substantially reduced or eliminated.